No Brain Too Small PHYSICS

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1 Level 3 Physics: Demonstrate understanding of mechanical systems Linear Mechanics - Answers In 03, AS 954 replaced AS 905. The Mess that is NCEA Assessment Schedules. In AS 905 there was an Evidence column with the correct answer and Achieved, Merit and Excellence columns explaining the required level of performance to get that grade. Each part of the question (row in the Assessment Schedule) contributed a single grade in either Criteria (Explain stuff) or Criteria (Solve stuff). From 003 to 008, the NCEA shaded columns that were not relevant to that question (Sorry haven t had time to do 004 yet). In 954, from 03 onwards, each part of a question contributes to the overall Grade Score Marking of the question and there are no longer separate criteria. There is no shading anymore. There is no spoon. At least their equation editor has stopped displaying random characters over the units. Question Evidence Achievement Merit Excellence 05(3) The centre of mass accelerates towards the ground as a single force (weight) acts upon it. Linear momentum increases due to the external force (weight), causing an increase in vertical velocity. Centre of mass accelerates downwards. Linear momentum increases because the vertical velocity is increasing. Centre of mass accelerates downwards. AND Linear momentum increases due to external force (weight) causing an increase in vertical velocity. (Accept Momentum increases because velocity increases and the net (external) force is not zero.) (Accept velocity is increasing.)

2 04(3) P = mv = evidence of correct number calculated e.g. p = mv mv + mv = (m + m)vcom pcom or TOTAL = pa + pb AND Substitution into equation to find correct answer. (b)(i) Momentum is conserved, so the size of the change of momentum on disc A is equal to the size of the change in momentum of disc B. As B had no momentum to start with, its final momentum must be equal to the change in momentum. Momentum is conserved. (In words, or equations, or by stating ) Statement of Newton 3rd law, Momentum is conserved (In words, or equations, or by stating ) Statement of Newton 3rd law, AND

3 (b)(ii) psystem = kg m s psystem = pa + pb p = mv equation used correctly for some momentum. Shows Pythagoras with momentum, wrong answer. Shows evidence of Pythagoras or vector diagram. AND ONLY if no credit given for or (b)(i). Correct vector diagram with labels or numbers - diagram either p or total p. (Arrows not needed) wrong answer. Carry on error from wrong p in 3a or 3b(i) OK. (c) The only forces that will be acting on the discs will be tension force in the cord. As this is internal force acting within the system, neither the momentum of the system nor the velocity of the centre of mass of the system will change. No friction therefore no external force acting on the system thus momentum is conserved. No friction / frictionless. No external forces. Tension / force on string is an internal force. Forces due to cord oppose / cancel. (Accept closed system in place of no external forces.) No friction, no external forces. No external forces because force due to string is internal. No external forces because string forces oppose / cancel each other. (Accept closed system in place of no external forces.) No friction therefore no external forces therefore p is conserved AND Force of the string is internal. String forces oppose / cancel each other. (Accept closed system in place of no external forces ) 0(3) Momentum is conserved in the horizontal direction because there are no external forces. Kinetic energy is not conserved because the elastic potential energy of the spring has been converted into kinetic energy / the kinetic energy of the system has increased. Momentum is conserved and kinetic energy is not conserved plus some idea why kinetic energy is not conserved. kinetic energy not conserved is linked to the explosion of the spring. Momentum is conserved and kinetic energy is not conserved because energy is gained from the spring. Full explanation for why kinetic energy is not conserved. Correct and full explanation.

4 (b) pbefore = pafter = mv = = = 7.04 kg m s (c) or Momentum vector diagram drawn and each vector labelled with variable name. or Momentum vector diagram drawn, values given for pa and p i, θa labelled. or Momentum vector diagram plus explanation of how to use it to find velocity. Momentum of particle A can be calculated: pa = mava or pa = = 6.40 Initial momentum equals final momentum calculated in (b). Correct method of finding pb using vectors, could use vector components / scale drawing of triangle / cosine rule. pa = mbvb, so knowing the mass of B from mball ma ie = 0.05 kg (d) Impulse is caused by the force of gravity. The effect of the impulse is to increase the vertical momentum but not the horizontal momentum, which means that the total momentum of the ball has increased. Impulse is caused by the force of gravity. Force of gravity causes an increase in vertical momentum. Impulse is caused by the force of gravity which means the vertical component of the velocity / momentum increases. The effect of the impulse is to increase the vertical momentum but not the horizontal momentum. Impulse is caused by the force of gravity and its effect is to change the final momentum of the ball by increasing its vertical component.

5 (e) Impulse = p y Initial p y is zero, so p y = impulse = 3. Initial Correct vertical velocity. Correct velocity Correct velocity and angle. (Question can be answered using momentum vectors.) 00() The only forces acting to bring about the change in position are internal forces. Good idea of no external forces acting. (b) Correct working (c) v = (3.5 ms +.8 m s ) = = = 3.94 m s

6 (d) F = ma = m v t, p = Ft F = m v t Correct acceleration / correct change in momentum / correct force using m = 55 kg. v = vi (vf = 0) F = = 3 N (e) p = Ft. The vertical speed is half the horizontal speed, and so change in vertical momentum is half the change in vertical momentum. However, the vertical stopping time is one tenth the horizontal stopping time and so the upward force needed to stop the vertical motion will be much greater than the horizontal friction force needed to stop the horizontal motion. Shortness of the vertical stopping time identified as the key issue. Correct calculation of average horizontal or vertical force. Short stopping time linked to the need for a large force. Answer shows understanding of how the difference in stopping times has greater significance than the difference in momentums. 009(4) vcofm = Σp/Σm =.3/( ) vcofm = 4. m s v = 4. m s v = 4.3 m s vcofm = ((9 x 0.46) + (0 x 0.65)) / 0.3 = 4.3 m s (b) Velocity of centre of mass remains constant because there are no external forces. Remains constant. Remains constant due to one of: no external forces or Momentum is conserved

7 (c) v i= 9.0 m s v f = 4.0 m s v = 9.85 m s v = (9 + 4 ) = 9.85 m s (d) p for ball = m v = =.438 kg m s p for ball = p for ball pi for ball = 0 pf for ball =.438 kg m s vf for ball =.438/ 0.65 = 8.7 m s Correct direction θ = 4.0 o Uses a vector diagram Vector diagram requires arrows Correct velocity and incorrect direction v = 8.69 to 8.7 m s Correct p of ball p = Correct velocity and direction v = 8.69 to 8.7 m s θ = 4.0 o Direction: θ tan θ = 4/9 θ = 4.0 o to the original direction of ball

8 (e) Sam only considered linear kinetic energy. In reality the balls also rotate, and so have rotational kinetic energy. During the collision, some rotational kinetic energy from ball could be transferred as linear kinetic energy, explaining the apparent increase in energy. Other possibilities could include an explosive device / way of adding energy from another form during the collision or having the table on a slope so that gpe was being converted to ke. Plausible reason stated E.g.: Kinetic: Rotation is the cause Gravitational: Sloping table or ball sitting on a bump Chemical: Explosive device of some type. Other: Ball under stress breaks apart. Plausible reason + explanation of the energy transfer. E.g.: Kinetic: Ek(rot) ball transferred to Ek(linear) ball. Gravitational: The Ep(grav) is transferred in to Ek(linear) Chemical: Chemical energy to Ek(linear) Other: Elastic energy to Ek(linear) Or any system where external energy is added to the system. An energy that is added should be from an external source except Ek(rot) 008() Forces correctly labelled Can be through COM. (should be roughly equal in length).

9 (b) Chair moves because of unbalanced torque. Any of: Reason for movement Reason for equilibrium Due to centre of mass to the right of the suspension cable / weight of skier causes torque. Torque is force distance between the line of action of the force and the pivot. Net torque causes angular acceleration. Hangs in new position where new centre of mass is below suspension point, so there is no net torque and no angular acceleration. COM moved to the right (towards X) The new COM now (moved to be) below the suspension point Weight of person causes a torque A couple: causes a torque Produces a torque Torque causes rotation of the chair (in clockwise direction) about pivot Gives reason why the chair is in equilibrium using torques (see below) Justifies why the COM is now under the pivot - balanced torque - no resultant torque 008(4) (c)(i) Momentum gained by the ball p = mv = = 0.8 kg m s, this equals momentum lost. Correct calculation of p (0.8 kgms - ) Correct direction Both magnitude and direction correct Accept either directions: p or momentum loss Accept either directions: p or momentum loss

10 (c)(ii) Initial p = = 8 kg m s to the right. Has incorrect vector diagram but uses it to give some explanation about why change is negligible Must use momentum diagram as part of their explanation (not just a velocity diagram) A correct consistent vector diagram or analysis (from 4c(i)) A correct vector diagram or analysis and uses correct explanation about why change negligible (c)(iii) Because (in the collision) the change of momentum of the club head and the golf ball is unchanged and the ball is much lighter, the gain in speed of the ball is much greater than the decrease in v of the club head. or The club and the head experience the same force. However as the mass of the club head is greater than the mass of the ball the acceleration of the ball will be higher (F = ma) Therefore the ball will travel faster States that momentum is conserved relates change of momentum of golf club head to golf balls velocity Mentions that any of these is the same in collision F force Ft impulse p change in mtm. on ball and club head 007() p = ( ) 5.4 = 9.45 Correct working

11 (b) Before the collision the jack is stationary, so the momentum of the bowl is the momentum of the system. Idea that the stationary jack is a key issue. Idea that the momentum of the system is the sum of the individual momentums, and that the momentum of the jack is zero. (c) dj = d m b Correct working. m b + m j = = (d) t = d v = = =.6 s (e) cosθ = θ =.98 = 3 (f) p(bowl) = Ft p(bowl) = = Correct answer shows recognition that the p of the bowl is the final p minus the initial p. Correct p(bowl). Correct answer F = = N = 50 N